Category: Calculators
Battery Life Calculator
Estimated runtime from capacity and load current
Enter a battery's capacity in milliamp-hours and your circuit's load current to estimate how many hours and days it will run.
An estimate only. Real runtime is usually shorter than the raw mAh ÷ mA division because of self-discharge, temperature, voltage sag under load, and the way many battery chemistries lose usable capacity at high discharge rates.
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What this tool does
Runtime is capacity divided by load current, adjusted by a usable-capacity percentage to account for the fact that real batteries deliver less than their rated capacity once discharge rate, temperature and voltage sag are taken into account.
How to use it
- Enter the battery's rated capacity in mAh, from its label or datasheet.
- Enter your circuit's average load current in mA.
- Adjust the usable-capacity percentage if you want a more conservative real-world estimate.
- Read the estimated runtime in hours and days underneath.
Privacy
This tool runs entirely in your browser. Your input is never uploaded, stored or shared — closing the tab removes it.
Frequently asked questions
- Why include a usable-capacity percentage instead of just dividing mAh by mA?
- A battery's printed capacity is measured under specific lab conditions. Real-world runtime is usually shorter because of self-discharge, temperature, voltage sag under load, and chemistries like alkaline losing a larger share of their capacity at higher discharge currents. A percentage below 100% gives a more realistic estimate.
- What usable-capacity percentage should I use?
- For a light, steady load from a quality lithium cell, 85 to 95% is a reasonable starting point. For a heavy load or a battery known to sag badly under load, such as alkaline cells at high current, 60 to 75% is more realistic. Testing your actual setup is the only way to know for certain.
- Does this account for standby or sleep-mode current?
- No, it assumes one constant load current for the whole runtime. For a device that alternates between an active and a sleep current, calculate a weighted average current first, based on how much time is spent in each state.